Use of estrogen replacement therapy (ERT) may increase a postmenopausal woman’s risk of ovarian cancer, according to findings published in the journal Cancer Research. Furthermore, ERT may advance breast cancer, which starts in lymph nodes located under the arms.

Researchers gave hormone treatments to mice that developed estrogen receptor positive ovarian cancer, a form of the disease that makes up 60 percent of all cases. These animals were then compared to another group with the disease, but didn’t undergo ERT. The team also compared two groups of mice with breast cancer, with one group receiving ERT.

The results showed that ovarian cancer spread five times faster in mice that had been exposed to ERT as opposed to the animals that didn’t receive the treatment. Also, ERT increased the risk of cancerous tumors developing in the lymph nodes by 26 percent, while the control group had a 6 percent increase.

Monique Spillman, lead author of the study, concluded that “we cannot make clinical recommendations based on what is happening in mice.” She added that “every woman is different and needs to talk to her doctor about the decision to use hormone replacement therapy.”

In 2006, nearly 20,000 women in the U.S. had been diagnosed with ovarian cancer, and of that population, a total of 14,857 patients died, according to the Centers for Disease Control and Prevention.